1. Field of the Invention
The invention relates to a semiconductor device and a fabrication method thereof.
2. Description of Related Art
With the progress in optoelectronic technologies, the fabrication and application of semiconductor devices have gradually matured. Devices such as optoelectronic devices and electronic devices can be fabricated with semiconductors, thus making the semiconductor industry a promising industry.
Generally speaking, the structure of a semiconductor device includes a substrate and at least one semiconductor layer disposed on the substrate. Considering the properties of lower cost, larger wafer size, higher thermal and electrical conductivities and the potential for integrating with electrical circuits, silicon substrates have been considered as one type of substrates for fabricating semiconductor light-emitting devices, and silicon (111) substrates are the most common type.
However, since the thermal expansion coefficient of the silicon substrate is lower than the thermal expansion coefficient of the gallium nitride semiconductor layer for fabricating the light-emitting device, there is a tensile stress generated in the nitride semiconductor layer during the temperature-dropping process after film-forming. The tensile stress drags the lattice structure of the nitride semiconductor layer, resulting in cracks of the nitride semiconductor layer. In addition, due to lattice mismatch between the nitride semiconductor layer and the silicon substrate, the nitride semiconductor layer grown on the silicon substrate tends to have a high density of threading dislocations. The phenomena described above influence the characteristics of the nitride semiconductor device. Therefore, issues such as reducing cracks and threading dislocations are certainly the issues that researchers nowadays need to work on.